It is known that hydrocarbon cracking processes are commonly employed in the petroleum and allied industries. They consist of breaking down the hydrocarbon molecules into smaller molecules by raising the temperature. There are two types of cracking, thermal cracking and catalytic cracking, which utilize either the effect of temperature alone or then the active sites of a catalyst.
In a conventional thermal cracking unit, the hydrocarbon feedstock is gradually heated in a tubular furnace. The thermal cracking reaction takes place mainly in the portion of the tubes receiving the maximum heat flow, where the temperature is determined by the nature of the hydrocarbons to be cracked.
In the visbreaking processes, in which only the heaviest molecules are broken down into smaller molecules, the cracking temperature ranges from 450.degree. to 600.degree. C., as the case may be.
When the molecules to be thermally cracked are lighter molecules, such as gasolines or liquefied petroleum gases (LPG), and light olefins and monoaromatic compounds are to be produced, the necessary temperature is much higher and generally ranges from 780.degree. to 850.degree. C., depending on the type of feedstock to be cracked, but is limited by the operating conditions of the process and by the operating complexity of the furnaces, which use supplementary heating energy.
Obtaining and maintaining the necessary temperature levels is all the more difficult as unwanted coke gradually deposits on the walls of the tubes and the heat flow is reduced. Moreover, a wall temperature that is higher than the process temperature accounts for the formation of coke and of breakdown products of the gum and acetylene-compound type. The coke detracts from the quality of the heat transfer. It results in a buildup of the pressure drop within the tubes and in an increase in the skin temperature which imposes excessive mechanical stresses that lower the conversion rate of the hydrocarbon feedstock entering the thermal cracking unit and entails periodic decoking outages. This also means that the process should be modular, to permit decoking on-stream, and that the feedstocks to be treated should be "clean" so that the duration of the cycles between two decoking operations is not too short. In practice, the feedstocks are limited to liquefied petroleum gas, gasolines, and certain well-suited or hydrotreated gas oils.
Moreover, the heat transfer within a tube is not instantaneous and the thermal cracking reaction is highly endothermic. This gives rise to problems of temperature control and maintenance, and hence of selectivity, which are very difficult to solve.
To overcome these drawbacks and carry out the thermal cracking of hydrocarbons, it has long been proposed to employ the fluidized-bed technique.
For example, U.S. Pat. No. 3,074,878 (Esso) and European patent application 26,674 (Stone) use a fluidized-phase tubular reactor with downflow of the heat-transfer particles, with a short contact time, to perform the thermal cracking of petroleum feedstocks, owing to a heat input supplied by the combustion of the coke deposited on the heat-transfer particles.
And U.S. Pat. No. 4,427,538 (Engelhard) employs a tubular reactor to carry out a low-severity cracking, and the elimination of the heaviest hydrocarbons contained in the feedstock, by means of a fluidized-phase reaction with upflow of inert heat-transfer particles.
However, none of these techniques is able to permit, under satisfactory industrial conditions, the simultaneous conversion to light olefins and to monoaromatic compounds of several fractions of petroleum hydrocarbons, such as liquefied petroleum gases, gasolines, or, much less, strongly contaminated residual feedstocks.
In fact, the thermal cracking of petroleum hydrocarbon fractions which include light paraffins such as butanes, propane, and especially ethane or of petroleum fractions such as gasolines, naphthas and gas oils requires that the reaction temperature be maintained at a very high level, generally on the order of from 750.degree. to 850.degree. C., for a very short but closely controlled time. In the absence of precise control of the residence time in that temperature zone, molecules of olefins formed during the conversion may polymerize to the detriment of the overall selectivity of the reaction. Now it has been found that the separation systems used up to now to separate the reaction effluents from the heat-transfer particles do not generally permit a sufficiently rapid separation and quenching of the effluents, with the result that some of the molecules formed may continue to react and to polymerize.
This poses a constant risk of clogging and fouling in the separating and stripping zone as well as of the piping for the effluent hydrocarbons between that zone and the fractionating zone.
Moreover, maintenance of the gas phase at the temperatures desired for thermal cracking requires an instantaneous and very substantial heat input because of the high endothermicity of the thermal conversion by steam cracking and because a rather sizable quantity of steam is injected into the reaction zone for the purpose of lowering the partial pressure of the hydrocarbons and of minimizing the production of coke. Furthermore, when light hydrocarbon fractions are being steam-cracked to olefins and monoaromatic compounds, the amount of coke deposited on the heat-transfer particles is altogether insufficient to maintain the heat balance of the system and makes necessary the systematic input of external energy.
Finally, the heat balance and the temperature level to be attained impose very high temperatures on the heat-transfer mass. Now because of technological difficulties due in part to the metallurgy of the equipment involved, only the most recent techniques permit the necessary quantities of heat-transfer particles to be provided at a sufficiently high temperature.